The present invention relates to frequency responses of microphones and particularly to changing the frequency response.
Microphones employ transducers, such as dynamic transducers, condenser transducers, electret transducers, solid state transducers, and other types of transducers, to convert impinging sound energy (pressure waves in air) into electrical signals which can be amplified and broadcast to an audience or applied to recording equipment to record a performance. Ideally, the electrical signals from the transducer are directly proportional to the sound energy arriving at the microphone at all frequencies across the audio spectrum, i.e., a flat frequency response from about 20 to 20,000 Hz. However all types of microphone transducers, and other microphone elements such as the microphone head affecting the sound energy, are mechanical devices which respond differently to different frequencies of sound energy and thus fail to produce a flat frequency response. Furthermore some microphones are intentionally designed to increase and/or decrease certain portions of the audio spectrum. Often equalizer circuits are employed to attenuate selected portions of the frequency spectrum in the electrical signals from microphones to increase the flatness of the response or to produce a desired change in the frequency response.
Different brands and types of commercially available microphones differ in frequency response. Often a musician prefers one brand and/or type of microphone that best suits his/her voice and style. Sometimes musicians use several different brands and types of microphones during a recording session or a performance to add color and variety to the performance. Concert hall engineers are often required to stock an extensive inventory of microphones or microphone heads so that they can accommodate the request of each artist who performs in their concert halls and studios. This is expensive especially for performers who require wireless microphones.
Additionally the mechanical portions of microphones must move in response to the impinging sound energy and, due to the inertia of these mechanical elements, the phase of the electrical signals produced by microphones varies with frequency. This microphone phase response is different for different brands and types of microphones. Although a different phase response is discernible to a lesser degree than a different frequency response, the sound reproduced and broadcast from the different microphones differs due to the different phase responses of the microphones.
There is a commercially available unit which can be plugged serially in a microphone cord for converting electrical signals from one brand or type of microphone to emulate another brand or type of microphone.
The invention is summarized in a circuit for changing electrical signals generated by a microphone into signals emulating the frequency response of another selected microphone by digitally filtering the microphone signals. The circuit includes an analog to digital converter which digitizes the microphone signals for processing by a digital signal processor based upon a set of processing parameters selected by a selector from a memory containing a plurality of sets of the processing parameters corresponding to different brands or types of microphones.
Each set of processing parameters is generated by a calibrated evaluation of the frequency response of two microphones, the microphone be used and the microphone being emulated. The differences between the frequency responses of the two microphones is used to produce the processing parameters such as digital filter parameters used by the digital signal processor to change the digitized electrical signals.
The emulation circuit is particularly useful when wireless microphones are employed. The emulation circuit can be incorporated in either the receiver or the wireless microphone itself